1. Field of the Invention
The present invention relates to a method of mixing fluids and a mixing apparatus adopting the method, and more particularly, to a method of mixing fluids by causing electrokinetic instability in a channel and a mixing apparatus adopting the method.
2. Description of the Related Art
Microfluidic devices that can perform chemical or biological analyses using a chip have received significant attention over the past decade. With the development of related technologies, the scales of these devices have decreased below 1 mm and various analysis devices which filled laboratories in the past can now be integrated onto a credit card-sized chip, which is called a “Lab-on-a-Chip”. Such technical progress has resulted in a reduction of production costs and has enabled various analysis experiments to be simultaneously performed, thereby reducing analysis time, reducing the amounts of samples used and allowing in situ operations. Thus, the Lab-on-a-Chip technology is expected to contribute greatly to the development of biomolecular research such as genomics, proteomics, etc.
In the miniaturization and integration of microfluidic devices, a variety of design parameters should be carefully considered. One important design parameter is that biological or biochemical reagents or solutions be homogeneously mixed within a limited time.
When the mixing time is similar to or greater than a chemical reaction time, rapid mixing becomes more important. In a microfluidic device, a capillary with a very small internal diameter is often used and a microfluid passing through the capillary has a very low Reynold's number. At a very low Reynold's number, laminar flow occurs, and thus the turbulence, which is very valuable as a stirring means, cannot be used, which makes rapid mixing difficult.
Homogeneous mixing is achieved when there is no concentration gradient. The reduction of a concentration gradient in laminar flow is largely dependent on molecular diffusion. The diffusion time tD is proportional to the square of a diffusion length LD as follows
                              t          D                ≈                              L            D            2                    D                                    (        1        )            where D is the diffusion coefficient.
Thus, to reduce the diffusion time for a constant diffusion coefficient, a method of increasing a contact boundary of two fluids mixed and reducing the diffusion distance is being developed. Mixing methods such as lamination mixing, micro-plume injection, chaotic mixing, parallel/serial mixing, and the like are known.
The lamination mixing is an effective mixing method, but requires a fine three-dimensional (3D) structure which has high production costs and requires a channel with a large cross-sectional area. Teachings on lamination mixing can be found in “Microfluidic Devices for Elecrokinetically Parallel and Serial Mixing”, Anal. Chem., 1999, 71, 4455-4459, by Jacobson et al., “A Modular Microfluid System with an Integrated Micromixer”, J. Micromech. Microeng. 1996, 6, 99-102, by Schwessinger, et al., U.S. Pat. No. 6,213,151, and U.S. Pat. No. 6,241,379. The parallel/serial mixing has similar problems to the lamination mixing and requires a long channel for sufficient mixing. The parallel/serial method is described by Jacobson, et al.
The microplume injection is a method of injecting fluid A into fluid B through multiple microplumes and the length of a channel required for mixing is relatively short. The fluid A injected into the fluid B slowly diffuses to be homogeneous. The homogeneity of the mixture is proportional to the number density of the microplumes into which the fluid A is injected per unit cross-sectional area. However, it is difficult to process the microplumes for injecting the fluid A. Microplume injection is described in detail in “Towards Integrated Microliquid Handling Systems”, J. MicroMech. Microeng. 1994, 4, 227-245, by Elwenspoek, et al.
The chaotic mixing is obtained through chaotic convection using a forced jet. However, to practically use chaotic mixing, a very complicated structure is required, and thus technical and economical difficulties arise. This method is describe in detail in “Chaotic Mixing in Electrokinetically and Pressure Driven Micro Flow”, Proc. 14th IEEE Workshop MEMS 2001, 483-486, by Lee et al., and “Passive Mixing in a Three-Dimensional Serpentine Microchannel”, J. Microelectromech. Syst. 2000, 9, 190-197, by Liu et al.
All of the above-described mixing methods are referred to as “passive mixing” methods which are differentiated from active mixing methods. In general, active mixing methods include an operating unit or an external mixing means such as pressure or an electric field. An active mixing method including the operating unit has difficulties in terms of molding and control of a mixing apparatus, and thus, is used only in special cases.
U.S. Pat. No. 6,086,243 issued to Paul et al. discloses a method of and an apparatus for effectively and rapidly mixing liquids in a creeping flow regime. According to Paul et al., fluids in a capillary which cannot be stirred mechanically or by turbulence can be homogeneously mixed by applying an electric field to each liquid. However, Paul et al. requires a separate chamber for mixing, thereby demanding more space and has low mixing efficiency due to the use of only circulation flow caused by a direct current (DC) power supplied to the liquid.
U.S. Pat. No. 6,482,306 issued to Yager et al. discloses an efficient apparatus for mixing liquids which does not require a separate chamber by forming electrodes and a chargeable surface on the wall surface of a channel. Yager et al. is more suitable for continuous flow than Paul et al., but discloses only circulation flow formed by supplying DC power, and thus is limited in terms of mixing efficiency.
U.S. Patent Application Publication No. 2002-125134 issued to Santiago et al. enhances mixing efficiency by supplying alternating current (AC) power instead of DC power. That is, when AC power is applied to both sides of a channel, arbitrary 3D fluctuations occur in a liquid within a few seconds, thereby causing electrokinetic instability (EKI) which stirs liquids actively, rapidly and effectively. A method of mixing a solution using EKI to obtain a homogeneous solution is useful in various fields, such as biochemistry, etc. However, in Santiago et al., a separate mixing chamber for supplying the AC power in a direction perpendicular to the flow direction of the fluid is required, which results in an unnecessary dead-zone. In addition, only the supply of the AC power is described, and how to optimize the AC power and maximize the mixing efficiency is not mentioned.
In addition, Santiago et al. attempted to mix two fluids by supplying DC power in a T-shaped channel. However, flow choking is caused at a point where two fluids meet and convective mixing no longer occurs due to laminar flow in a downstream of the channel. When the intensity of the electric field is increased to solve these problems, electrolysis or the formation of bubbles takes place.